rollup merge of #19454: nodakai/libstd-reap-failed-child
After the library successfully called `fork(2)`, the child does several
setup works such as setting UID, GID and current directory before it
calls `exec(2)`. When those setup works failed, the child exits but the
parent didn't call `waitpid(2)` and left it as a zombie.
This patch also add several sanity checks. They shouldn't make any
noticeable impact to runtime performance.
The new test case in `libstd/io/process.rs` calls the ps command to check
if the new code can really reap a zombie.
The output of `ps -A -o pid,sid,command` should look like this:
where `./spawn-failure` is my test program which intentionally leaves many
zombies. Filtering the output with the "SID" (session ID) column is
a quick way to tell if a process (zombie) was spawned by my own test
program. Then the number of "defunct" lines is the number of zombie
children.